The present invention relates to a path calculating system for following the path of a naval vessel with the help of magnetic means. It is particularly usable for tracking a vessel which is immunized against the risks of magnetic detection, particularly in stations which check the magnetic immunization of the vessel.
In a magnetic immunization checking station, measurements are made on a ship or a submarine moving above an aligned array of sensors for, among other things, adjusting the immunization loop currents. For this, the position of the vessel must be located with respect to the sensors for each measurement so as to be able to compare the results of measurements from several passages of the vessel.
The improvements made to magnetic detection systems have led to designing more reliable immunization systems allowing the vessels to escape from a magnetic detection system. Thus, it is necessary to accurately test the immunization of the vessel with high precision, and to be able to locate the current vessel position very accurately during such tests.
Until recently, and in numerous cases still at the present time, the means for locating the were essentially manual; the visual sighting of posts either by an operator or by a video image link. At the present time, the positioning and path calculating systems are radioelectric or optical; they consist in performing a triangulation from a mobile marker or beacon (on the vessel) and two or three fixed markers or beacons disposed on land.
Such an assembly is very large with respect to the size of the measuring base and the corrections to be performed; in fact, the site (normally a port) requires ranges of the order of 4 to 5 kilometers;
The positioning accuracy actually attained is on the order of 2 m in the best of cases. Such an installation is therefore very large. It requires an expensive infrastructure (construction, supply beacon, maintenance, etc.).
The path of a surface craft or a submarine can be obtained acoustically. At least three acoustic sensors or hydrophones are placed and fixed to the sea bed. The vessel whose path is to be calculated is equipped with an acoustic transmitter. Measurement of the times at which the emitted signal is received at the hydrophones allows the position of the craft to be calculated. This technique gives good measuring accuracy but requires a specific acoustic base (3 hydrophones) to be set up.
The present apparatus invention provides for calculating the path of a vessel which apparatus is sufficiently accurate, easy to implement and inexpensive.